Light-emitting devices containing network electrode polymers in electron blocking layer

ABSTRACT

In general terms, the present invention includes a light emitting polymeric material, the polymeric material comprising: (a) an electron transporting polymer; the electron transporting polymer in contact with (b) an electron blocking polymer, the electron blocking polymer incorporating a network electrode polymer. Such devices may be bilayer or multilayer devices, in accordance with arrangements known in the art. Likewise, the source of electrical current may be from any appropriate source having the electrical characteristics sufficient to and appropriate for the desired device make-up and application.

This application claims the benefit of U.S. Provisional Application Ser.No. 60/023,071 filed Aug. 2, 1996.

TECHNICAL FIELD

The present invention is in the field of light-emitting polymers andlight emitting devices produced therefrom.

BACKGROUND

Since the report in 1990 of electroluminescence (EL) in poly(ρ-phenylenevinylene) (PPV) [1], EL of conjugated polymers has been considered animportant property with many potential applications. Electroluminescencecombined with other unique properties of polymers, such as solutionprocessibility, band gap tunability, and mechanical flexibility, makeconjugated polymers excellent candidates for low cost large area displayapplications. In addition to PPV, a variety of PPV derivatives and otherconjugated polymers and copolymers have been found to exhibitelectroluminescent properties [2,3]. Light-emitting devicesincorporating these materials have demonstrated all the necessary colorsneeded for display applications.

Since the initial fabrication, a number of techniques have beendeveloped to improve the device performance. One way is to use a lowworkfunction metal, such as Ca, as the electron injecting electrode(cathode) [4]. The double charge injection mechanism of polymerlight-emitting diodes (LEDs) requires the match of cathode (anode)workfunction to the corresponding LUMO (HOMO) level of the polymer inorder to achieve efficient charge injection. The relatively smallelectron affinity of most conjugated polymers requires metals with verylow workfunctions to achieve efficient electron injection. However,since low workfunction metals are generally oxygen reactive, deviceswith low workfunction cathode are usually unstable. Thus, polymers withhigh electron affinities are desirable.

Another common technique is to incorporate charge transporting layers ina multilayer device structure. The charge transporting layer enhancesthe transport of one type of charge while blocking the other, achievingbalanced charge injection and transport and spatially confined emissionzone away from the electrodes. To date the highest efficiency polymerlight-emitting devices reported are multilayer devices [5].

Pyridine-based conjugated polymers have been shown to be promisingcandidates for light-emitting devices [6,7]. As compared tophenylene-based analogues, one of the most important features of thepyridine based polymers is the higher electron affinity. As aconsequence, the polymer is more resistant to oxidation and shows betterelectron transport properties. In contrast, most other conjugatedpolymers are susceptible to oxidation and exhibit better hole transportproperties. FIG. 1 shows the structures of the pyridine-containingpolymers and copolymers, namely poly(ρ-pyridine) (PPy), poly(p-pyridylvinylene) (PPyV), and copolymers of PPyV and PPV (PPyVP(R)₂ V) withvarious functional sidegroups R=C₁₂ H₂₅, OC₁₆ H₃₃, COOC₁₂ H₂₅. Withrespect to π electronic levels, C₁₂ H₂₅ is slightly electron donating;OC₁₆ H₃₃ electron donating; and COOC₁₂ H₂₅ electron withdrawing. Thepyridine-based polymers are highly luminescent, especially thecopolymers. The internal photoluminescent quantum efficiencies of thecopolymers have been measured [8] to be 75-90% in solution and 18-30% infilm, with the exception of the OC₁₆ H₃₃ copolymer. The electrondonating nature of OC₁₆ H₃₃ makes this copolymer more susceptible foroxidation. As a result, the PL quantum efficiency of the OC₁₆ H₃₃copolymer is only 2% in film although it is high (˜80%) in solution. Toreduce the oxidation effects, the strapped copolymer (@PPyVPV) wasintroduced, as shown in FIG. 1(d). Also the strapped copolymer showsfewer aggregation effects as compared to the "usual" copolymers (seeFIG. 1).

It is an object of the present invention to improve the performance oflight-emitting polymers, such as reducing the required voltage required,and thus achieving similar levels of brightness while reducing theamount of power required for electroluminescence.

In view of the present disclosure or through practice of the presentinvention, other advantages may become apparent.

SUMMARY OF THE INVENTION

In general terms, the present invention includes a light emittingpolymeric material, the polymeric material comprising: (a) an electrontransporting polymer; the electron transporting polymer in contact with(b) an electron blocking polymer, the electron blocking polymerincorporating a network electrode polymer. Such devices may be bilayeror multilayer devices, in accordance with arrangements known in the art.Likewise, the source of electrical current may be from any appropriatesource having the electrical characteristics sufficient to andappropriate for the desired device make-up and application.

The electron transporting polymer may be any conductive polymericmaterial of appropriate conductive and electron affinity characteristicsto allow it to act as the electron transporting polymer in a lightemitting device. Examples of such polymers include pyridine-containingconjugated polymers and copolymers, and their derivatives. Likewise, theelectron blocking polymer may be any polymeric material of appropriateelectron-blocking characteristics to allow it to act as the electronblocking polymer in a light emitting device, such as those selected fromthe group consisting of poly(vinylcarbazoles) and their derivatives.

The network electrode polymer may be any polymeric material that formsan electrically conducting network polymeric structure within theelectron blocking polymer. Examples include camphor sulfonic acid dopedpolyanilines. The network electrode polymers of the present inventionmay be produced through methods known in the art such as those used inthe synthesis of extended π-systems [17] and in the synthesis of ladderpolymers [18].

The present invention also includes light emitting devices incorporatinglight emitting polymeric materials of the present invention. In generalterms, such devices comprise: (a) an electron transporting polymer; theelectron transporting polymer in contact with (b) an electron blockingpolymer, the electron blocking polymer incorporating a network electrodepolymer; and (c) a source of electrical current so as to supply theelectron transporting polymer with a flow of electrons, so as to causean electroluminescent emission from the heterojunction between theelectron transporting polymer and the electron blocking polymer.

In accordance with the present invention, there is disclosedlight-emitting devices based on the pyridine-containing polymers andcopolymers in various device configurations. The high electron affinityof pyridine based polymers enables the use of relatively stable metalssuch as A1 or even ITO as electron injecting contacts. Taking advantageof the better electron transport properties of the pyridine-containingpolymers, we fabricate bilayer devices utilizing poly(9-vinyl carbazole)(PVK) as hole transporting/electron blocking polymer, which improves thedevice efficiency and brightness significantly due to the chargeconfinement and exciplex emission at the PVK/emitting polymer interface.The incorporation of conducting polyaniline network electrode to PVKreduces the device turn on voltage significantly while maintaining thehigh efficiency. The control of the aggregation in the polymer films byblending with insulating host polymers open up the possibility of makingvoltage-controlled multi-color light-emitting devices. The capability ofeliminating the use of low workfunction metals makes the pyridine basedpolymers an excellent candidate for polymer light-emitting devices.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 shows the chemical structures of pyridine-based conjugatedpolymers and copolymers: (a) poly(p-pyridine) (PPy), (b) poly(p-pyridylvinylene) (PPyV), (c) copolymers of PPyV and PPV derivatives (PPyVP(R)₂V) with various functional sidegroups R=C₁₂ H₂₅, OC₁₆ H₃₃, COOC₁₂ H₂₅,and (d) strapped copolymer (@PPyVPV).

FIG. 2 shows a schematic structure of a bilayer device with conductingpolyaniline network electrode in accordance with one embodiment of thepresent invention.

FIG. 3 shows a normalized optical absorption (dashed line) and PL of thestrapped copolymer film (solid line), EL of a single layer device (solidline with dots), and PL of solution in xylenes (dotted line).

FIG. 4 shows a comparison of (a) light-voltage and (b) light-currentcharacteristics for a single layer device (square), a bilayer device(circle), and a bilayer device with PAN-CSA network (triangle). Inset:EL spectra for the single layer device (dashed line), the bilayer device(solid line), and the bilayer device with network electrode (dottedline).

FIG. 5 shows a film PL of the pure wrapped copolymer and its blends withPMMA in various ratios with an excitation energy of 2.65 eV, andsolution PL of the copolymer in xylenes. Inset: Film PL of a 1:20 blendwith different excitation energies as indicated in the graph. Spectraare offset for clarity.

FIG. 6 shows a schematic structure of an inverted light-emitting deviceswith PPy as emitting layer and PVK as hole transporting layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the foregoing summary of the invention, the followingpresents a detailed description of the preferred embodiment of theinvention which is presently considered to be its best mode.

The synthesis of the pyridine-containing polymers has been reportedearlier [9-10]. For single layer devices, the emitting layer wasspin-cast from solutions in formic acid (for PPy and PPyV) or xylenes(for copolymers) (with a concentration ˜10 mg/ml) onto pre-cleanedpatterned ITO substrates with sheet resistance of ˜15 Ω/square at1000-2000 rpm. For bilayer devices, PVK layer was spin coated onto ITOsubstrate from solution in tetrahydrofuran (THF) (˜10 mg/ml) at ˜3000rpm. The emitting layer was then spin coated on top of the PVK layerfrom appropriate solutions. The conducting polyaniline network electrodewas formed by a spin-cast blend of camphor sulfonic acid dopedpolyaniline (PAN-CSA) and low molecular weight host polymer poly(methylmethacrylate) (PMMA) (from Aldrich Chemical Co.) in an appropriate ratioin m-cresol. The host polymer PMMA was subsequently washed away byxylenes. The PVK and emitting layers were similarly coated as in thebilayer device. All solutions were filtered using Gelman Acrodisc CRPTFE 1 μm filters. The top metal electrode was deposited by vacuumevaporation at a pressure below 10⁻⁶ torr. To prevent damage to thepolymers, the substrate was mounted on a cold-water cooled surfaceduring evaporation. FIG. 2 shows schematically the structure of abilayer device with PAN-CSA network electrode.

Absorption spectra were measured on spin-cast films using a Perkin-ElmerLambda 19 UVNVIS/NIR spectrometer. Photoluminescence (PL) and EL weremeasured using a PTI fluorometer (model QM-1). The current-voltage (I-V)characteristics were measured simultaneously with EL using two Keithley195A multimeters while dc voltage was applied by a HP 6218A DC powersupply.

FIG. 3 shows the optical absorption and PL of the strapped copolymerfilm and EL of a single layer device. For comparison, the PL of thestrapped copolymer solution in xylenes is also shown. The film PL peaksat 2.05 eV with a shoulder at 2.25 eV. As compared to the filmabsorbance, the peak of the film PL is redshifted 0.55 eV, which isattributed to the aggregates formed in the film [12]. The shoulder issuggested to come from the unaggregated site, and is supported by the PLmeasurements of blends in PMMA (see below). It is noted that althoughthe strapped and the corresponding unstrapped copolymer show similarfeatures in solution PL, no shoulder is found in the film PL for theunstrapped copolymer, indicating that the strapped side chains partiallybreak the aggregates formation in the film. The reversed oscillatorstrength of the EL as compared to PL suggesting that the EL come mainlyfrom unaggregated sites, although there is also a significantcontribution from the aggregate emission.

FIG. 4 compares the light-voltage (L-V) and EL-current (EL-I)characteristics for a single layer device, a bilayer device, and abilayer device with PAN-CSA network electrode using the strappedcopolymer as emitting layer. As compared to those of the single layerdevice, the quantum efficiency and brightness of the bilayer deviceincrease more than two orders of magnitude, reaching ˜0.3% and ˜300cd/m² respectively. PVK is a well known hole transporting/electronblocking polymer. Besides the function of enhancing the transport ofholes injected from anode, it blocks the transport of electrons injectedfrom cathode such that the electrons accumulate at the PVK/copolymerinterface. This greatly enhance the probability of radiativerecombination. In addition, the PVK layer separates the recombinationzone from the metal electrode so that the radiative recombination isprotected against the well known non-radiative quenching at themetal/polymer interfaces.

One side effect of using the PVK layer is that it increases the deviceoperating voltage substantially. One effective way to reduce the deviceturn on voltage is to use high surface network electrode [13]. Theconcept behind the network electrode is that a rough electrode willcreate a non-uniform high electric field that enhances the chargeinjection. This technique has been successfully applied to PPV baseddevices [13]. By applying this technique to the PVK layer, the deviceoperating voltage decreased significantly. For the devices shown here,the device operating voltage reduced from ˜20 V to ˜8 V (see FIG. 4(a)).Since the incorporation of the PAN-CSA network electrode does not modifythe PVK/copolymer interface, the high quantum efficiency and brightnessof the bilayer device are maintained (see FIG. 4(b)). Thus, theincorporation of the network electrode to the bilayer device improvesthe power efficiency dramatically.

The species that is responsible for the light generation in the bilayerdevice is attributed partially to exciplexes formed at the PVK/copolymerinterface and is identified by the PL measurements [14]. FIG. 4(b) insetcompares the EL spectra of a single and a bilayer device using thestrapped copolymer as emitting layer. As compare to that of the singlelayer device, the peak of the bilayer device, which comes from theexciplex emission at the PVK/copolymer interface, is blue-shifted 0.15eV. A shoulder in the bilayer EL at the peak of the single layer ELsuggests that the strapped copolymer EL itself also contribute to thebilayer EL.

The large difference between the film and solution PL of thepyridine-based polymers opens up an opportunity for fabricatingvoltage-controlled color-variable light-emitting devices. The aggregatesformed in the polymer films result in significantly red-shiftedluminescence as compared to isolated chains in solution. One expects toreduce the red-shift of PL by breaking the aggregates formation. Oneeffective way to break the aggregation is to blend the emissive polymerwith an insulating host polymer, such as in PMMA. FIG. 5 shows the PLspectra of the pure wrapped copolymer and its blends with PMMA invarious ratios. For comparison, the PL spectrum of the wrapped copolymerin solution is also shown. When the concentration of the emissivepolymer decreases, the PL of the blends gradually blue shifted towardsthe solution PL, indicating partial break of the aggregation of polymerchains. Thus by choosing appropriate blend ratio, the emission color canbe controlled. Furthermore, the PL spectra of the blends exhibitexcitation energy dependence, as shown in FIG. 5 inset for a blend with1:20 (copolymer:PMMA) ratio excited at different energies. As theexcitation energy increases, the PL strength of the higher energy peakgrows. In contrast, no excitation energy dependence is found in purecopolymer PL. The excitation energy dependence of the blend PL make itpossible to fabricate voltage controlled multi-color light-emittingdevices, and the work is in progress.

The high electron affinity of the pyridine-based polymers enables othernovel device configurations such as inverted light-emitting devices thatare capable of eliminating the use of low workfunction metals.Poly(ρ-pyridine) (PPy) has an electron affinity of ˜3.5 eV [16], whichallows metals with relatively high workfunction as electron injectingcontact. In the inverted light-emitting devices with PPy as emissivelayer, ITO and Au are used as electron and hole injecting contacts,respectively. The inverted (-)ITO/PPy/Au(+) device show improved deviceperformance including quantum efficiency, brightness, operatingstability and storage lifetimes as compared to the usual(+)ITO/PPy/Al(-) device. By inserting a PVK layer in between the PPy andAu, the device performance improves further FIG. 6 shows schematicallythe device structure of the inverted light-emitting device with PVK.

Conclusion

In summary, pyridine containing conjugated polymers and copolymers areexcellent candidates for polymer light-emitting devices. The highelectron affinity of pyridine based polymers enables the use ofrelatively stable metals such as Al or even ITO as efficient electroninjecting contacts. Taking advantages of the better electron transportproperties of the pyridine-containing polymers, we fabricate bilayerdevices utilizing PVK as hole transporting/electron blocking polymer.The bilayer device structure improves the device quantum efficiency andbrightness significantly due to the charge confinement and the exciplexemission at the PVK/emitting polymer interface. The incorporation of theconducting polyaniline network electrode to PVK reduces the device turnon voltage significantly while maintaining the high efficiency andbrightness of the bilayer device. The control of the aggregation in thepolymer films by blending with insulating host polymers opens up thepossibility of making voltage-controlled multi-color light-emittingdevices.

The following references are hereby incorporated herein by reference:

References

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The contents of U.S. Provisional Patent Application Ser. No. 60/023,071are hereby incorporated herein by reference.

In view of the present disclosure or through practice of the presentinvention, it will be within the ability of one of rodinary skill tomake modifications to the present invention, such as through the use ofequivalent arrangements and compositions, in order to practice theinvention without departing from the spirit of the invention asreflected in the appended claims.

What is claimed is:
 1. A light emitting polymeric material, saidpolymeric material comprising:(a) an electron transporting polymer; saidelectron transporting polymer in contact with (b) an electron blockingpolymer, said electron blocking polymer incorporating a networkelectrode polymer.
 2. A light emitting polymeric material according toclaim 1, wherein said electron tranporting polymer comprises apyridine-containing conjugated polymers and copolymers.
 3. A lightemitting polymeric material according to claim 1, wherein said electronblocking polymer is selected from the group consisting ofpoly(vinylcarbazole).
 4. A light emitting polymeric material accordingto claim 1, wherein said network electrode polymer comprises camphorsulfonic acid doped polyaniline.
 5. A light emitting device, said devicecomprising:(a) an electron transporting polymer; said electrontransporting polymer in contact with (b) an electron blocking polymer,said electron blocking polymer incorporating a network electrodepolymer; and (c) a source of electrical current so as to supply saidelectron transporting polymer with a flow of electrons, so as to causean electroluminescent emission from said heterojunction between saidelectron transporting polymer and said electron blocking polymer.
 6. Alight emitting device according to claim 5, wherein said electrontranporting polymer comprises a pyridine-containing conjugated polymersand copolymers.
 7. A light emitting device according to claim 5, whereinsaid electron blocking polymer is selected from the group consisting ofpoly(vinylcarbazole).
 8. A light emitting device according to claim 5,wherein said network electrode polymer comprises camphor sulfonic aciddoped polyaniline.